The aiming and firing of missiles and projectiles from a maneuvering airborne weapon platform such as a military aircraft involves the computation of and comparison of the predicted trajectories of the target and the missile to determine necessary steering changes to be applied in the control of the weapon platform. Such fire control technique conventionally employs the line-of-sight range to the target, and the direction angle components of the line-of-sight direction, as determined by a radar system. In such application, the radar system may preferrably be of the tracking type, whereby the radar antenna boresight axis is servoed or otherwise maintained substantially coincident with the line of sight (L.O.S.) to the target.
The target position information thus provided by the radar is expressed in the polar coordinates of the antenna which may be rotating relative to the platform on which it is mounted, which platform itself may be rotating relative to inertial space. Hence, the computation of predicted future target positions employing time derivative or rate-of-change data involves the differentiation of a vector quantity measured in a rotating coordinate system. Such computation has involved the computation of cross products of angular rates. Also, such differentiation process magnifies noise content of the signals (as is well understood) as to require low pass filtering to attenuate such noise. Prediction of target acceleration (usually termed second order) has not been practiced for many airborne systems because of noise problems.
Prior art smoothing techniques in fire control prediction computation, using tracking radars, have effected vector filtering by means of rate gyros on the radar antenna. Forms of such techniques are described for exmple in U.S. Pat. No. 3,123,822 issued to R. G. Shelley, et al, for Filter for Information Expressed in Rotating Coordinates; U.S. Pat. No. 3,185,817 issued to R. G. Shelley for Gyroscope Filtering and Computing System; and U.S. Pat. No. 2,805,022 issued to R. G. Shelley for Vector Filter System. Such techniques serve to smooth the target position and rate gyro data provided by the radar. Because such position and rate gyro data represents the effect of both ownship motion and target motion, such smoothing inserts a lag in ownship data. In other words, the radar tracking filter is in the tracking loop, as to inert the filter lag in the tracking loop as a contribution to fire-control errors and tends to decrease tracking stability of the pilot. Also, the use of such rate gyros contributes to the angular errors due to the inherent thresholds and non-linearities of operation and the output noise levels of such devices. The mechanization of such gyroscope filters, by means including instrument servos of limited accuracy and limited dynamic response, contributes additional noise and error. Such noise levels in general prevent the use of second order prediction techniques as to limit the theoretical accuracies obtainable.
Radar track mechanizations currently used require high dynamic response to stabilize against aircraft maneuver, thereby making the radar more susceptable to ECM equipment and techniques.
As military aircraft speeds increase, it is necessary to fire the weapons of such arcraft at increased ranges and with reduced reaction time. Such increased ranges in conjunction with allowable missdistances thus define allowable angular steering errors of decreasing magnitudes. Such smaller allowable angular steering errors thus tend to require greater accuracies than those obtainable from the rate gyro state-of-the-art, and tend to make intolerable that time-lag imposed on ownship motion data due to inclusion of the radar prediction filter in the overall geometry-control loop. Also, such need for increased accuracy tends to require the use of second order prediction techniques rather than more approximate methods of fire control computation for the short range delivery of missiles against maneuvering targets.